/*
 * Copyright 2002-2021 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.servlet;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.beans.factory.BeanFactoryUtils;
import org.springframework.beans.factory.BeanInitializationException;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.i18n.LocaleContext;
import org.springframework.core.MethodParameter;
import org.springframework.core.annotation.AnnotationAwareOrderComparator;
import org.springframework.core.io.ClassPathResource;
import org.springframework.core.io.support.PropertiesLoaderUtils;
import org.springframework.core.log.LogFormatUtils;
import org.springframework.http.HttpMethod;
import org.springframework.http.server.RequestPath;
import org.springframework.http.server.ServletServerHttpRequest;
import org.springframework.lang.Nullable;
import org.springframework.ui.context.ThemeSource;
import org.springframework.util.ClassUtils;
import org.springframework.util.StringUtils;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.context.request.ServletWebRequest;
import org.springframework.web.context.request.async.WebAsyncManager;
import org.springframework.web.context.request.async.WebAsyncUtils;
import org.springframework.web.method.annotation.ModelFactory;
import org.springframework.web.method.support.ModelAndViewContainer;
import org.springframework.web.multipart.MultipartException;
import org.springframework.web.multipart.MultipartHttpServletRequest;
import org.springframework.web.multipart.MultipartResolver;
import org.springframework.web.servlet.function.support.HandlerFunctionAdapter;
import org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver;
import org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter;
import org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter;
import org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter;
import org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor;
import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;
import org.springframework.web.servlet.support.SessionFlashMapManager;
import org.springframework.web.servlet.view.InternalResourceViewResolver;
import org.springframework.web.servlet.view.RedirectView;
import org.springframework.web.util.NestedServletException;
import org.springframework.web.util.ServletRequestPathUtils;
import org.springframework.web.util.WebUtils;

import javax.servlet.DispatcherType;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.*;
import java.util.stream.Collectors;

/**
 * Central dispatcher for HTTP request handlers/controllers, e.g. for web UI controllers
 * or HTTP-based remote service exporters. Dispatches to registered handlers for processing
 * a web request, providing convenient mapping and exception handling facilities.
 *
 * <p>This servlet is very flexible: It can be used with just about any workflow, with the
 * installation of the appropriate adapter classes. It offers the following functionality
 * that distinguishes it from other request-driven web MVC frameworks:
 *
 * <ul>
 * <li>It is based around a JavaBeans configuration mechanism.
 *
 * <li>It can use any {@link HandlerMapping} implementation - pre-built or provided as part
 * of an application - to control the routing of requests to handler objects. Default is
 * {@link org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping} and
 * {@link org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping}.
 * HandlerMapping objects can be defined as beans in the servlet's application context,
 * implementing the HandlerMapping interface, overriding the default HandlerMapping if
 * present. HandlerMappings can be given any bean name (they are tested by type).
 *
 * <li>It can use any {@link HandlerAdapter}; this allows for using any handler interface.
 * Default adapters are {@link org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter},
 * {@link org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter}, for Spring's
 * {@link org.springframework.web.HttpRequestHandler} and
 * {@link org.springframework.web.servlet.mvc.Controller} interfaces, respectively. A default
 * {@link org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter}
 * will be registered as well. HandlerAdapter objects can be added as beans in the
 * application context, overriding the default HandlerAdapters. Like HandlerMappings,
 * HandlerAdapters can be given any bean name (they are tested by type).
 *
 * <li>The dispatcher's exception resolution strategy can be specified via a
 * {@link HandlerExceptionResolver}, for example mapping certain exceptions to error pages.
 * Default are
 * {@link org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver},
 * {@link org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver}, and
 * {@link org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver}.
 * These HandlerExceptionResolvers can be overridden through the application context.
 * HandlerExceptionResolver can be given any bean name (they are tested by type).
 *
 * <li>Its view resolution strategy can be specified via a {@link ViewResolver}
 * implementation, resolving symbolic view names into View objects. Default is
 * {@link org.springframework.web.servlet.view.InternalResourceViewResolver}.
 * ViewResolver objects can be added as beans in the application context, overriding the
 * default ViewResolver. ViewResolvers can be given any bean name (they are tested by type).
 *
 * <li>If a {@link View} or view name is not supplied by the user, then the configured
 * {@link RequestToViewNameTranslator} will translate the current request into a view name.
 * The corresponding bean name is "viewNameTranslator"; the default is
 * {@link org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator}.
 *
 * <li>The dispatcher's strategy for resolving multipart requests is determined by a
 * {@link org.springframework.web.multipart.MultipartResolver} implementation.
 * Implementations for Apache Commons FileUpload and Servlet 3 are included; the typical
 * choice is {@link org.springframework.web.multipart.commons.CommonsMultipartResolver}.
 * The MultipartResolver bean name is "multipartResolver"; default is none.
 *
 * <li>Its locale resolution strategy is determined by a {@link LocaleResolver}.
 * Out-of-the-box implementations work via HTTP accept header, cookie, or session.
 * The LocaleResolver bean name is "localeResolver"; default is
 * {@link org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver}.
 *
 * <li>Its theme resolution strategy is determined by a {@link ThemeResolver}.
 * Implementations for a fixed theme and for cookie and session storage are included.
 * The ThemeResolver bean name is "themeResolver"; default is
 * {@link org.springframework.web.servlet.theme.FixedThemeResolver}.
 * </ul>
 *
 * <p><b>NOTE: The {@code @RequestMapping} annotation will only be processed if a
 * corresponding {@code HandlerMapping} (for type-level annotations) and/or
 * {@code HandlerAdapter} (for method-level annotations) is present in the dispatcher.</b>
 * This is the case by default. However, if you are defining custom {@code HandlerMappings}
 * or {@code HandlerAdapters}, then you need to make sure that a corresponding custom
 * {@code RequestMappingHandlerMapping} and/or {@code RequestMappingHandlerAdapter}
 * is defined as well - provided that you intend to use {@code @RequestMapping}.
 *
 * <p><b>A web application can define any number of DispatcherServlets.</b>
 * Each servlet will operate in its own namespace, loading its own application context
 * with mappings, handlers, etc. Only the root application context as loaded by
 * {@link org.springframework.web.context.ContextLoaderListener}, if any, will be shared.
 *
 * <p>As of Spring 3.1, {@code DispatcherServlet} may now be injected with a web
 * application context, rather than creating its own internally. This is useful in Servlet
 * 3.0+ environments, which support programmatic registration of servlet instances.
 * See the {@link #DispatcherServlet(WebApplicationContext)} javadoc for details.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @author Rob Harrop
 * @author Chris Beams
 * @author Rossen Stoyanchev
 * @author Sebastien Deleuze
 * @see org.springframework.web.HttpRequestHandler
 * @see org.springframework.web.servlet.mvc.Controller
 * @see org.springframework.web.context.ContextLoaderListener
 */
@SuppressWarnings("serial")
public class DispatcherServlet extends FrameworkServlet {

    /**
     * Well-known name for the MultipartResolver object in the bean factory for this namespace.
     */
    public static final String MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver";

    /**
     * Well-known name for the LocaleResolver object in the bean factory for this namespace.
     */
    public static final String LOCALE_RESOLVER_BEAN_NAME = "localeResolver";

    /**
     * Well-known name for the ThemeResolver object in the bean factory for this namespace.
     */
    public static final String THEME_RESOLVER_BEAN_NAME = "themeResolver";

    /**
     * Well-known name for the HandlerMapping object in the bean factory for this namespace.
     * Only used when "detectAllHandlerMappings" is turned off.
     *
     * @see #setDetectAllHandlerMappings
     */
    public static final String HANDLER_MAPPING_BEAN_NAME = "handlerMapping";

    /**
     * Well-known name for the HandlerAdapter object in the bean factory for this namespace.
     * Only used when "detectAllHandlerAdapters" is turned off.
     *
     * @see #setDetectAllHandlerAdapters
     */
    public static final String HANDLER_ADAPTER_BEAN_NAME = "handlerAdapter";

    /**
     * Well-known name for the HandlerExceptionResolver object in the bean factory for this namespace.
     * Only used when "detectAllHandlerExceptionResolvers" is turned off.
     *
     * @see #setDetectAllHandlerExceptionResolvers
     */
    public static final String HANDLER_EXCEPTION_RESOLVER_BEAN_NAME = "handlerExceptionResolver";

    /**
     * Well-known name for the RequestToViewNameTranslator object in the bean factory for this namespace.
     */
    public static final String REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME = "viewNameTranslator";

    /**
     * Well-known name for the ViewResolver object in the bean factory for this namespace.
     * Only used when "detectAllViewResolvers" is turned off.
     *
     * @see #setDetectAllViewResolvers
     */
    public static final String VIEW_RESOLVER_BEAN_NAME = "viewResolver";

    /**
     * Well-known name for the FlashMapManager object in the bean factory for this namespace.
     */
    public static final String FLASH_MAP_MANAGER_BEAN_NAME = "flashMapManager";

    /**
     * Request attribute to hold the current web application context.
     * Otherwise only the global web app context is obtainable by tags etc.
     *
     * @see org.springframework.web.servlet.support.RequestContextUtils#findWebApplicationContext
     */
    public static final String WEB_APPLICATION_CONTEXT_ATTRIBUTE = DispatcherServlet.class.getName() + ".CONTEXT";

    /**
     * Request attribute to hold the current LocaleResolver, retrievable by views.
     *
     * @see org.springframework.web.servlet.support.RequestContextUtils#getLocaleResolver
     */
    public static final String LOCALE_RESOLVER_ATTRIBUTE = DispatcherServlet.class.getName() + ".LOCALE_RESOLVER";

    /**
     * Request attribute to hold the current ThemeResolver, retrievable by views.
     *
     * @see org.springframework.web.servlet.support.RequestContextUtils#getThemeResolver
     */
    public static final String THEME_RESOLVER_ATTRIBUTE = DispatcherServlet.class.getName() + ".THEME_RESOLVER";

    /**
     * Request attribute to hold the current ThemeSource, retrievable by views.
     *
     * @see org.springframework.web.servlet.support.RequestContextUtils#getThemeSource
     */
    public static final String THEME_SOURCE_ATTRIBUTE = DispatcherServlet.class.getName() + ".THEME_SOURCE";

    /**
     * Name of request attribute that holds a read-only {@code Map<String,?>}
     * with "input" flash attributes saved by a previous request, if any.
     *
     * @see org.springframework.web.servlet.support.RequestContextUtils#getInputFlashMap(HttpServletRequest)
     */
    public static final String INPUT_FLASH_MAP_ATTRIBUTE = DispatcherServlet.class.getName() + ".INPUT_FLASH_MAP";

    /**
     * Name of request attribute that holds the "output" {@link FlashMap} with
     * attributes to save for a subsequent request.
     *
     * @see org.springframework.web.servlet.support.RequestContextUtils#getOutputFlashMap(HttpServletRequest)
     */
    public static final String OUTPUT_FLASH_MAP_ATTRIBUTE = DispatcherServlet.class.getName() + ".OUTPUT_FLASH_MAP";

    /**
     * Name of request attribute that holds the {@link FlashMapManager}.
     *
     * @see org.springframework.web.servlet.support.RequestContextUtils#getFlashMapManager(HttpServletRequest)
     */
    public static final String FLASH_MAP_MANAGER_ATTRIBUTE = DispatcherServlet.class.getName() + ".FLASH_MAP_MANAGER";

    /**
     * Name of request attribute that exposes an Exception resolved with a
     * {@link HandlerExceptionResolver} but where no view was rendered
     * (e.g. setting the status code).
     */
    public static final String EXCEPTION_ATTRIBUTE = DispatcherServlet.class.getName() + ".EXCEPTION";

    /**
     * Log category to use when no mapped handler is found for a request.
     */
    public static final String PAGE_NOT_FOUND_LOG_CATEGORY = "org.springframework.web.servlet.PageNotFound";

    /**
     * Name of the class path resource (relative to the DispatcherServlet class)
     * that defines DispatcherServlet's default strategy names.
     */
    private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";

    /**
     * Common prefix that DispatcherServlet's default strategy attributes start with.
     */
    private static final String DEFAULT_STRATEGIES_PREFIX = "org.springframework.web.servlet";

    /**
     * Additional logger to use when no mapped handler is found for a request.
     */
    protected static final Log pageNotFoundLogger = LogFactory.getLog(PAGE_NOT_FOUND_LOG_CATEGORY);

    /**
     * Store default strategy implementations.
     */
    @Nullable
    private static Properties defaultStrategies;

    /**
     * Detect all HandlerMappings or just expect "handlerMapping" bean?.
     */
    private boolean detectAllHandlerMappings = true;

    /**
     * Detect all HandlerAdapters or just expect "handlerAdapter" bean?.
     */
    private boolean detectAllHandlerAdapters = true;

    /**
     * Detect all HandlerExceptionResolvers or just expect "handlerExceptionResolver" bean?.
     */
    private boolean detectAllHandlerExceptionResolvers = true;

    /**
     * Detect all ViewResolvers or just expect "viewResolver" bean?.
     */
    private boolean detectAllViewResolvers = true;

    /**
     * Throw a NoHandlerFoundException if no Handler was found to process this request? *.
     */
    private boolean throwExceptionIfNoHandlerFound = false;

    /**
     * Perform cleanup of request attributes after include request?.
     */
    private boolean cleanupAfterInclude = true;

    /**
     * MultipartResolver used by this servlet.
     */
    @Nullable
    private MultipartResolver multipartResolver;

    /**
     * LocaleResolver used by this servlet.
     */
    @Nullable
    private LocaleResolver localeResolver;

    /**
     * ThemeResolver used by this servlet.
     */
    @Nullable
    private ThemeResolver themeResolver;

    /**
     * List of HandlerMappings used by this servlet.
     */
    @Nullable
    private List<HandlerMapping> handlerMappings;

    /**
     * List of HandlerAdapters used by this servlet.
     */
    @Nullable
    private List<HandlerAdapter> handlerAdapters;

    /**
     * List of HandlerExceptionResolvers used by this servlet.
     */
    @Nullable
    private List<HandlerExceptionResolver> handlerExceptionResolvers;

    /**
     * RequestToViewNameTranslator used by this servlet.
     */
    @Nullable
    private RequestToViewNameTranslator viewNameTranslator;

    /**
     * FlashMapManager used by this servlet.
     */
    @Nullable
    private FlashMapManager flashMapManager;

    /**
     * List of ViewResolvers used by this servlet.
     */
    @Nullable
    private List<ViewResolver> viewResolvers;

    private boolean parseRequestPath;


    /**
     * Create a new {@code DispatcherServlet} that will create its own internal web
     * application context based on defaults and values provided through servlet
     * init-params. Typically used in Servlet 2.5 or earlier environments, where the only
     * option for servlet registration is through {@code web.xml} which requires the use
     * of a no-arg constructor.
     * <p>Calling {@link #setContextConfigLocation} (init-param 'contextConfigLocation')
     * will dictate which XML files will be loaded by the
     * {@linkplain #DEFAULT_CONTEXT_CLASS default XmlWebApplicationContext}
     * <p>Calling {@link #setContextClass} (init-param 'contextClass') overrides the
     * default {@code XmlWebApplicationContext} and allows for specifying an alternative class,
     * such as {@code AnnotationConfigWebApplicationContext}.
     * <p>Calling {@link #setContextInitializerClasses} (init-param 'contextInitializerClasses')
     * indicates which {@code ApplicationContextInitializer} classes should be used to
     * further configure the internal application context prior to refresh().
     *
     * @see #DispatcherServlet(WebApplicationContext)
     */
    public DispatcherServlet() {
        super();
        setDispatchOptionsRequest(true);
    }

    /**
     * Create a new {@code DispatcherServlet} with the given web application context. This
     * constructor is useful in Servlet 3.0+ environments where instance-based registration
     * of servlets is possible through the {@link ServletContext#addServlet} API.
     * <p>Using this constructor indicates that the following properties / init-params
     * will be ignored:
     * <ul>
     * <li>{@link #setContextClass(Class)} / 'contextClass'</li>
     * <li>{@link #setContextConfigLocation(String)} / 'contextConfigLocation'</li>
     * <li>{@link #setContextAttribute(String)} / 'contextAttribute'</li>
     * <li>{@link #setNamespace(String)} / 'namespace'</li>
     * </ul>
     * <p>The given web application context may or may not yet be {@linkplain
     * ConfigurableApplicationContext#refresh() refreshed}. If it has <strong>not</strong>
     * already been refreshed (the recommended approach), then the following will occur:
     * <ul>
     * <li>If the given context does not already have a {@linkplain
     * ConfigurableApplicationContext#setParent parent}, the root application context
     * will be set as the parent.</li>
     * <li>If the given context has not already been assigned an {@linkplain
     * ConfigurableApplicationContext#setId id}, one will be assigned to it</li>
     * <li>{@code ServletContext} and {@code ServletConfig} objects will be delegated to
     * the application context</li>
     * <li>{@link #postProcessWebApplicationContext} will be called</li>
     * <li>Any {@code ApplicationContextInitializer}s specified through the
     * "contextInitializerClasses" init-param or through the {@link
     * #setContextInitializers} property will be applied.</li>
     * <li>{@link ConfigurableApplicationContext#refresh refresh()} will be called if the
     * context implements {@link ConfigurableApplicationContext}</li>
     * </ul>
     * If the context has already been refreshed, none of the above will occur, under the
     * assumption that the user has performed these actions (or not) per their specific
     * needs.
     * <p>See {@link org.springframework.web.WebApplicationInitializer} for usage examples.
     *
     * @param webApplicationContext the context to use
     * @see #initWebApplicationContext
     * @see #configureAndRefreshWebApplicationContext
     * @see org.springframework.web.WebApplicationInitializer
     */
    public DispatcherServlet(WebApplicationContext webApplicationContext) {
        super(webApplicationContext);
        setDispatchOptionsRequest(true);
    }


    /**
     * Set whether to detect all HandlerMapping beans in this servlet's context. Otherwise,
     * just a single bean with name "handlerMapping" will be expected.
     * <p>Default is "true". Turn this off if you want this servlet to use a single
     * HandlerMapping, despite multiple HandlerMapping beans being defined in the context.
     */
    public void setDetectAllHandlerMappings(boolean detectAllHandlerMappings) {
        this.detectAllHandlerMappings = detectAllHandlerMappings;
    }

    /**
     * Set whether to detect all HandlerAdapter beans in this servlet's context. Otherwise,
     * just a single bean with name "handlerAdapter" will be expected.
     * <p>Default is "true". Turn this off if you want this servlet to use a single
     * HandlerAdapter, despite multiple HandlerAdapter beans being defined in the context.
     */
    public void setDetectAllHandlerAdapters(boolean detectAllHandlerAdapters) {
        this.detectAllHandlerAdapters = detectAllHandlerAdapters;
    }

    /**
     * Set whether to detect all HandlerExceptionResolver beans in this servlet's context. Otherwise,
     * just a single bean with name "handlerExceptionResolver" will be expected.
     * <p>Default is "true". Turn this off if you want this servlet to use a single
     * HandlerExceptionResolver, despite multiple HandlerExceptionResolver beans being defined in the context.
     */
    public void setDetectAllHandlerExceptionResolvers(boolean detectAllHandlerExceptionResolvers) {
        this.detectAllHandlerExceptionResolvers = detectAllHandlerExceptionResolvers;
    }

    /**
     * Set whether to detect all ViewResolver beans in this servlet's context. Otherwise,
     * just a single bean with name "viewResolver" will be expected.
     * <p>Default is "true". Turn this off if you want this servlet to use a single
     * ViewResolver, despite multiple ViewResolver beans being defined in the context.
     */
    public void setDetectAllViewResolvers(boolean detectAllViewResolvers) {
        this.detectAllViewResolvers = detectAllViewResolvers;
    }

    /**
     * Set whether to throw a NoHandlerFoundException when no Handler was found for this request.
     * This exception can then be caught with a HandlerExceptionResolver or an
     * {@code @ExceptionHandler} controller method.
     * <p>Note that if {@link org.springframework.web.servlet.resource.DefaultServletHttpRequestHandler}
     * is used, then requests will always be forwarded to the default servlet and a
     * NoHandlerFoundException would never be thrown in that case.
     * <p>Default is "false", meaning the DispatcherServlet sends a NOT_FOUND error through the
     * Servlet response.
     *
     * @since 4.0
     */
    public void setThrowExceptionIfNoHandlerFound(boolean throwExceptionIfNoHandlerFound) {
        this.throwExceptionIfNoHandlerFound = throwExceptionIfNoHandlerFound;
    }

    /**
     * Set whether to perform cleanup of request attributes after an include request, that is,
     * whether to reset the original state of all request attributes after the DispatcherServlet
     * has processed within an include request. Otherwise, just the DispatcherServlet's own
     * request attributes will be reset, but not model attributes for JSPs or special attributes
     * set by views (for example, JSTL's).
     * <p>Default is "true", which is strongly recommended. Views should not rely on request attributes
     * having been set by (dynamic) includes. This allows JSP views rendered by an included controller
     * to use any model attributes, even with the same names as in the main JSP, without causing side
     * effects. Only turn this off for special needs, for example to deliberately allow main JSPs to
     * access attributes from JSP views rendered by an included controller.
     */
    public void setCleanupAfterInclude(boolean cleanupAfterInclude) {
        this.cleanupAfterInclude = cleanupAfterInclude;
    }


    /**
     * This implementation calls {@link #initStrategies}.
     */
    @Override
    protected void onRefresh(ApplicationContext context) {
        // 初始化策略
        initStrategies(context);
    }

    /**
     * Initialize the strategy objects that this servlet uses.
     * <p>May be overridden in subclasses in order to initialize further strategy objects.
     */
    protected void initStrategies(ApplicationContext context) {
        /**
         * 从容器中拿到 MultipartResolver bean，暴露给当前实例，拿不到就算了。
         * */
        initMultipartResolver(context);
        /**
         * 从容器中拿到 LocaleResolver bean，拿不到就读取 DispatcherServlet.properties 属性文件，
         * 然后使用 {@link AbstractAutowireCapableBeanFactory#createBean(Class)}
         * 创建出bean，然后暴露给当前实例（只允许有一个）
         * */
        initLocaleResolver(context);
        /**
         * 逻辑同上。从容器中拿到 ThemeResolver bean （只允许有一个）
         * */
        initThemeResolver(context);
        /**
         * 逻辑同上。从容器中拿到 HandlerMapping bean（多个）
         * */
        initHandlerMappings(context);
        /**
         * 逻辑同上。从容器中拿到 HandlerAdapter bean（多个）
         * */
        initHandlerAdapters(context);
        /**
         * 逻辑同上。从容器中拿到 HandlerExceptionResolver bean（多个）
         * */
        initHandlerExceptionResolvers(context);
        /**
         * 逻辑同上。从容器中拿到 RequestToViewNameTranslator bean（只允许有一个）
         * */
        initRequestToViewNameTranslator(context);
        /**
         * 逻辑同上。从容器中拿到 ViewResolver bean（多个）
         * */
        initViewResolvers(context);
        /**
         * 逻辑同上。从容器中拿到 FlashMapManager bean（只允许有一个）
         * */
        initFlashMapManager(context);
    }

    /**
     * Initialize the MultipartResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * no multipart handling is provided.
     */
    private void initMultipartResolver(ApplicationContext context) {
        try {
            this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.multipartResolver);
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.multipartResolver.getClass()
                        .getSimpleName());
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // Default is no multipart resolver.
            this.multipartResolver = null;
            if (logger.isTraceEnabled()) {
                logger.trace("No MultipartResolver '" + MULTIPART_RESOLVER_BEAN_NAME + "' declared");
            }
        }
    }

    /**
     * Initialize the LocaleResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * we default to AcceptHeaderLocaleResolver.
     */
    private void initLocaleResolver(ApplicationContext context) {
        try {
            this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.localeResolver);
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.localeResolver.getClass()
                        .getSimpleName());
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.localeResolver = getDefaultStrategy(context, LocaleResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "No LocaleResolver '" + LOCALE_RESOLVER_BEAN_NAME + "': using default [" + this.localeResolver.getClass()
                                .getSimpleName() + "]");
            }
        }
    }

    /**
     * Initialize the ThemeResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * we default to a FixedThemeResolver.
     */
    private void initThemeResolver(ApplicationContext context) {
        try {
            this.themeResolver = context.getBean(THEME_RESOLVER_BEAN_NAME, ThemeResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.themeResolver);
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.themeResolver.getClass()
                        .getSimpleName());
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.themeResolver = getDefaultStrategy(context, ThemeResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "No ThemeResolver '" + THEME_RESOLVER_BEAN_NAME + "': using default [" + this.themeResolver.getClass()
                                .getSimpleName() + "]");
            }
        }
    }

    /**
     * Initialize the HandlerMappings used by this class.
     * <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,
     * we default to BeanNameUrlHandlerMapping.
     */
    private void initHandlerMappings(ApplicationContext context) {
        this.handlerMappings = null;

        if (this.detectAllHandlerMappings) {
            // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerMapping> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(
                    context, HandlerMapping.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerMappings = new ArrayList<>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerMappings);
            }
        } else {
            try {
                HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
                this.handlerMappings = Collections.singletonList(hm);
            } catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we'll add a default HandlerMapping later.
            }
        }

        // Ensure we have at least one HandlerMapping, by registering
        // a default HandlerMapping if no other mappings are found.
        if (this.handlerMappings == null) {
            this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "No HandlerMappings declared for servlet '" + getServletName() + "': using default strategies from DispatcherServlet.properties");
            }
        }

        for (HandlerMapping mapping : this.handlerMappings) {
            if (mapping.usesPathPatterns()) {
                this.parseRequestPath = true;
                break;
            }
        }
    }

    /**
     * Initialize the HandlerAdapters used by this class.
     * <p>If no HandlerAdapter beans are defined in the BeanFactory for this namespace,
     * we default to SimpleControllerHandlerAdapter.
     */
    private void initHandlerAdapters(ApplicationContext context) {
        this.handlerAdapters = null;

        if (this.detectAllHandlerAdapters) {
            // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerAdapter> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(
                    context, HandlerAdapter.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerAdapters = new ArrayList<>(matchingBeans.values());
                // We keep HandlerAdapters in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerAdapters);
            }
        } else {
            try {
                HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
                this.handlerAdapters = Collections.singletonList(ha);
            } catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we'll add a default HandlerAdapter later.
            }
        }

        // Ensure we have at least some HandlerAdapters, by registering
        // default HandlerAdapters if no other adapters are found.
        if (this.handlerAdapters == null) {
            this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "No HandlerAdapters declared for servlet '" + getServletName() + "': using default strategies from DispatcherServlet.properties");
            }
        }
    }

    /**
     * Initialize the HandlerExceptionResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * we default to no exception resolver.
     */
    private void initHandlerExceptionResolvers(ApplicationContext context) {
        this.handlerExceptionResolvers = null;

        if (this.detectAllHandlerExceptionResolvers) {
            // Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(
                    context, HandlerExceptionResolver.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
                // We keep HandlerExceptionResolvers in sorted order.
                AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
            }
        } else {
            try {
                HandlerExceptionResolver her = context.getBean(
                        HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class);
                this.handlerExceptionResolvers = Collections.singletonList(her);
            } catch (NoSuchBeanDefinitionException ex) {
                // Ignore, no HandlerExceptionResolver is fine too.
            }
        }

        // Ensure we have at least some HandlerExceptionResolvers, by registering
        // default HandlerExceptionResolvers if no other resolvers are found.
        if (this.handlerExceptionResolvers == null) {
            this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "No HandlerExceptionResolvers declared in servlet '" + getServletName() + "': using default strategies from DispatcherServlet.properties");
            }
        }
    }

    /**
     * Initialize the RequestToViewNameTranslator used by this servlet instance.
     * <p>If no implementation is configured then we default to DefaultRequestToViewNameTranslator.
     */
    private void initRequestToViewNameTranslator(ApplicationContext context) {
        try {
            this.viewNameTranslator = context.getBean(
                    REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME, RequestToViewNameTranslator.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.viewNameTranslator.getClass()
                        .getSimpleName());
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.viewNameTranslator);
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.viewNameTranslator = getDefaultStrategy(context, RequestToViewNameTranslator.class);
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "No RequestToViewNameTranslator '" + REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME + "': using default [" + this.viewNameTranslator.getClass()
                                .getSimpleName() + "]");
            }
        }
    }

    /**
     * Initialize the ViewResolvers used by this class.
     * <p>If no ViewResolver beans are defined in the BeanFactory for this
     * namespace, we default to InternalResourceViewResolver.
     */
    private void initViewResolvers(ApplicationContext context) {
        this.viewResolvers = null;

        if (this.detectAllViewResolvers) {
            // Find all ViewResolvers in the ApplicationContext, including ancestor contexts.
            Map<String, ViewResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(
                    context, ViewResolver.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.viewResolvers = new ArrayList<>(matchingBeans.values());
                // We keep ViewResolvers in sorted order.
                AnnotationAwareOrderComparator.sort(this.viewResolvers);
            }
        } else {
            try {
                ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class);
                this.viewResolvers = Collections.singletonList(vr);
            } catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we'll add a default ViewResolver later.
            }
        }

        // Ensure we have at least one ViewResolver, by registering
        // a default ViewResolver if no other resolvers are found.
        if (this.viewResolvers == null) {
            this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "No ViewResolvers declared for servlet '" + getServletName() + "': using default strategies from DispatcherServlet.properties");
            }
        }
    }

    /**
     * Initialize the {@link FlashMapManager} used by this servlet instance.
     * <p>If no implementation is configured then we default to
     * {@code org.springframework.web.servlet.support.DefaultFlashMapManager}.
     */
    private void initFlashMapManager(ApplicationContext context) {
        try {
            this.flashMapManager = context.getBean(FLASH_MAP_MANAGER_BEAN_NAME, FlashMapManager.class);
            if (logger.isTraceEnabled()) {
                logger.trace("Detected " + this.flashMapManager.getClass()
                        .getSimpleName());
            } else if (logger.isDebugEnabled()) {
                logger.debug("Detected " + this.flashMapManager);
            }
        } catch (NoSuchBeanDefinitionException ex) {
            // We need to use the default.
            this.flashMapManager = getDefaultStrategy(context, FlashMapManager.class);
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "No FlashMapManager '" + FLASH_MAP_MANAGER_BEAN_NAME + "': using default [" + this.flashMapManager.getClass()
                                .getSimpleName() + "]");
            }
        }
    }

    /**
     * Return this servlet's ThemeSource, if any; else return {@code null}.
     * <p>Default is to return the WebApplicationContext as ThemeSource,
     * provided that it implements the ThemeSource interface.
     *
     * @return the ThemeSource, if any
     * @see #getWebApplicationContext()
     */
    @Nullable
    public final ThemeSource getThemeSource() {
        return (getWebApplicationContext() instanceof ThemeSource ? (ThemeSource) getWebApplicationContext() : null);
    }

    /**
     * Obtain this servlet's MultipartResolver, if any.
     *
     * @return the MultipartResolver used by this servlet, or {@code null} if none
     * (indicating that no multipart support is available)
     */
    @Nullable
    public final MultipartResolver getMultipartResolver() {
        return this.multipartResolver;
    }

    /**
     * Return the configured {@link HandlerMapping} beans that were detected by
     * type in the {@link WebApplicationContext} or initialized based on the
     * default set of strategies from {@literal DispatcherServlet.properties}.
     * <p><strong>Note:</strong> This method may return {@code null} if invoked
     * prior to {@link #onRefresh(ApplicationContext)}.
     *
     * @return an immutable list with the configured mappings, or {@code null}
     * if not initialized yet
     * @since 5.0
     */
    @Nullable
    public final List<HandlerMapping> getHandlerMappings() {
        return (this.handlerMappings != null ? Collections.unmodifiableList(this.handlerMappings) : null);
    }

    /**
     * Return the default strategy object for the given strategy interface.
     * <p>The default implementation delegates to {@link #getDefaultStrategies},
     * expecting a single object in the list.
     *
     * @param context           the current WebApplicationContext
     * @param strategyInterface the strategy interface
     * @return the corresponding strategy object
     * @see #getDefaultStrategies
     */
    protected <T> T getDefaultStrategy(ApplicationContext context, Class<T> strategyInterface) {
        List<T> strategies = getDefaultStrategies(context, strategyInterface);
        if (strategies.size() != 1) {
            throw new BeanInitializationException(
                    "DispatcherServlet needs exactly 1 strategy for interface [" + strategyInterface.getName() + "]");
        }
        return strategies.get(0);
    }

    /**
     * Create a List of default strategy objects for the given strategy interface.
     * <p>The default implementation uses the "DispatcherServlet.properties" file (in the same
     * package as the DispatcherServlet class) to determine the class names. It instantiates
     * the strategy objects through the context's BeanFactory.
     *
     * @param context           the current WebApplicationContext
     * @param strategyInterface the strategy interface
     * @return the List of corresponding strategy objects
     */
    @SuppressWarnings("unchecked")
    protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
        if (defaultStrategies == null) {
            try {
                // Load default strategy implementations from properties file.
                // This is currently strictly internal and not meant to be customized
                // by application developers.
                ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
                defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
            } catch (IOException ex) {
                throw new IllegalStateException("Could not load '" + DEFAULT_STRATEGIES_PATH + "': " + ex.getMessage());
            }
        }

        String key = strategyInterface.getName();
        String value = defaultStrategies.getProperty(key);
        if (value != null) {
            String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
            List<T> strategies = new ArrayList<>(classNames.length);
            for (String className : classNames) {
                try {
                    Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
                    Object strategy = createDefaultStrategy(context, clazz);
                    strategies.add((T) strategy);
                } catch (ClassNotFoundException ex) {
                    throw new BeanInitializationException(
                            "Could not find DispatcherServlet's default strategy class [" + className + "] for interface [" + key + "]",
                            ex
                    );
                } catch (LinkageError err) {
                    throw new BeanInitializationException(
                            "Unresolvable class definition for DispatcherServlet's default strategy class [" + className + "] for interface [" + key + "]",
                            err
                    );
                }
            }
            return strategies;
        } else {
            return Collections.emptyList();
        }
    }

    /**
     * Create a default strategy.
     * <p>The default implementation uses
     * {@link org.springframework.beans.factory.config.AutowireCapableBeanFactory#createBean}.
     *
     * @param context the current WebApplicationContext
     * @param clazz   the strategy implementation class to instantiate
     * @return the fully configured strategy instance
     * @see org.springframework.context.ApplicationContext#getAutowireCapableBeanFactory()
     * @see org.springframework.beans.factory.config.AutowireCapableBeanFactory#createBean
     */
    protected Object createDefaultStrategy(ApplicationContext context, Class<?> clazz) {
        return context.getAutowireCapableBeanFactory()
                .createBean(clazz);
    }


    /**
     * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
     * for the actual dispatching.
     */
    @Override
    protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        logRequest(request);

        // include 的处理，主要是处理完请求之后恢复属性信息
        // Keep a snapshot of the request attributes in case of an include,
        // to be able to restore the original attributes after the include.
        Map<String, Object> attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap<>();
            Enumeration<?> attrNames = request.getAttributeNames();
            while (attrNames.hasMoreElements()) {
                String attrName = (String) attrNames.nextElement();
                // 暂存这些属性
                if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                    attributesSnapshot.put(attrName, request.getAttribute(attrName));
                }
            }
        }

        // 设置 IOC容器、localeResolver、themeResolver、ThemeSource 到request域中
        // Make framework objects available to handlers and view objects.
        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

        /**
         * FlashMap 是用来记录重定向的数据的，原理就是数据存到session中
         * {@link SessionFlashMapManager}
         * */
        if (this.flashMapManager != null) {
            // 从session中获取
            FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
            if (inputFlashMap != null) {
                request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
            }
            /**
             * 设置 输入输出 FlashMap
             *
             * 在执行完 HandlerMethod 之后，会将RedirectAttributes的内容写到 输出FlashMap中
             * {@link RequestMappingHandlerAdapter#getModelAndView(ModelAndViewContainer, ModelFactory, NativeWebRequest)}
             *
             * 在这两个地方会将 内容写到session中
             *  {@link HttpEntityMethodProcessor#handleReturnValue(Object, MethodParameter, ModelAndViewContainer, NativeWebRequest)}
             *  {@link RedirectView#renderMergedOutputModel(Map, HttpServletRequest, HttpServletResponse)}
             * */
            request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
            request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
        }

        RequestPath previousRequestPath = null;
        if (this.parseRequestPath) {
            // 拿到之前的值
            previousRequestPath = (RequestPath) request.getAttribute(ServletRequestPathUtils.PATH_ATTRIBUTE);
            // 设置到 request域中
            ServletRequestPathUtils.parseAndCache(request);
        }

        try {
            // 进行转发
            doDispatch(request, response);
        } finally {
            if (!WebAsyncUtils.getAsyncManager(request)
                    .isConcurrentHandlingStarted()) {
                // 恢复属性信息
                // Restore the original attribute snapshot, in case of an include.
                if (attributesSnapshot != null) {
                    restoreAttributesAfterInclude(request, attributesSnapshot);
                }
            }
            if (this.parseRequestPath) {
                // 恢复属性信息
                ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
            }
        }
    }

    private void logRequest(HttpServletRequest request) {
        LogFormatUtils.traceDebug(logger, traceOn -> {
            String params;
            if (StringUtils.startsWithIgnoreCase(request.getContentType(), "multipart/")) {
                params = "multipart";
            } else if (isEnableLoggingRequestDetails()) {
                params = request.getParameterMap()
                        .entrySet()
                        .stream()
                        .map(entry -> entry.getKey() + ":" + Arrays.toString(entry.getValue()))
                        .collect(Collectors.joining(", "));
            } else {
                params = (request.getParameterMap()
                        .isEmpty() ? "" : "masked");
            }

            String queryString = request.getQueryString();
            String queryClause = (StringUtils.hasLength(queryString) ? "?" + queryString : "");
            String dispatchType = (!DispatcherType.REQUEST.equals(
                    request.getDispatcherType()) ? "\"" + request.getDispatcherType() + "\" dispatch for " : "");
            String message = (dispatchType + request.getMethod() + " \"" + getRequestUri(
                    request) + queryClause + "\", parameters={" + params + "}");

            if (traceOn) {
                List<String> values = Collections.list(request.getHeaderNames());
                String headers = values.size() > 0 ? "masked" : "";
                if (isEnableLoggingRequestDetails()) {
                    headers = values.stream()
                            .map(name -> name + ":" + Collections.list(request.getHeaders(name)))
                            .collect(Collectors.joining(", "));
                }
                return message + ", headers={" + headers + "} in DispatcherServlet '" + getServletName() + "'";
            } else {
                return message;
            }
        });
    }

    /**
     * Process the actual dispatching to the handler.
     * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
     * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
     * to find the first that supports the handler class.
     * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
     * themselves to decide which methods are acceptable.
     *
     * @param request  current HTTP request
     * @param response current HTTP response
     * @throws Exception in case of any kind of processing failure
     */
    @SuppressWarnings("deprecation")
    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        // 装饰了 映射的方法 + HandlerInterceptor(方法的拦截器)
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            ModelAndView mv = null;
            Exception dispatchException = null;

            try {
                // 检查请求体内容 是不是文件上传的
                processedRequest = checkMultipart(request);
                // 不相等，说明 是
                multipartRequestParsed = (processedRequest != request);

                /**
                 * 得到 HandlerExecutionChain
                 * 遍历属性 {@link DispatcherServlet#handlerMappings} 执行 {@link HandlerMapping#getHandler(HttpServletRequest)}
                 * 返回值不为null就返回。默认的HandlerMapping是执行的这个重写的方法 {@link AbstractHandlerMapping#getHandler(HttpServletRequest)}
                 *
                 * 默认是有这三个HandlerMapping：
                 *      - BeanNameUrlHandlerMapping
                 *      - RequestMappingHandlerMapping
                 *      - RouterFunctionMapping
                 * */
                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) {
                    // 找不到，默认是 设置状态码 404 不会抛异常
                    noHandlerFound(processedRequest, response);
                    return;
                }

                /**
                 * 根据 handler 获取适配器
                 * 遍历属性 {@link DispatcherServlet#handlerAdapters} 判断是否 {@link HandlerAdapter#supports(Object)}适配，true就返回该适配器
                 *
                 * 默认是有这四个：
                 *      - HttpRequestHandlerAdapter：处理 handler instanceof HttpRequestHandler
                 *      - SimpleControllerHandlerAdapter：处理 handler instanceof Controller
                 *      - RequestMappingHandlerAdapter：处理 handler instanceof HandlerMethod
                 *      - HandlerFunctionAdapter：处理 handler instanceof HandlerFunction
                 * */
                // Determine handler adapter for the current request.
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler.
                String method = request.getMethod();
                boolean isGet = HttpMethod.GET.matches(method);
                if (isGet || HttpMethod.HEAD.matches(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    /**
                     * 没有变化 就快速返回
                     * */
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }

                /**
                 * 回调 preHandle。
                 * 就是遍历 HandlerExecutionChain 中的 HandlerInterceptor 回调 {@link HandlerInterceptor#preHandle(HttpServletRequest, HttpServletResponse, Object)}
                 * 只要有一个返回false，那么就不需要执行后面的 HandlerInterceptor，依次回调已经执行的 HandlerInterceptor 的 afterCompletion 方法
                 * 然后快速返回false
                 * */
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                /**
                 * 使用适配器执行 handler
                 *
                 * 默认是有这四个：
                 *      - HttpRequestHandlerAdapter：处理 handler instanceof HttpRequestHandler
                 *          {@link HttpRequestHandlerAdapter#handle(HttpServletRequest, HttpServletResponse, Object)}
                 *          很简单，反射调用方法
                 *
                 *      - SimpleControllerHandlerAdapter：处理 handler instanceof Controller
                 *          {@link SimpleControllerHandlerAdapter#handle(HttpServletRequest, HttpServletResponse, Object)}
                 *          很简单，反射调用方法
                 *
                 *      - HandlerFunctionAdapter：处理 handler instanceof HandlerFunction
                 *          {@link HandlerFunctionAdapter#handle(HttpServletRequest, HttpServletResponse, Object)}
                 *          很简单，反射调用方法，然后拷贝内容到输出流
                 *
                 *      - RequestMappingHandlerAdapter：处理 handler instanceof HandlerMethod
                 *          这个就是用来处理 @RequestMapping 的，过程比较复杂，涉及到 @SessionAttributes 的属性获取，@ModelAttribute、@InitBinder 的方法回调,
                 *          方法参数列表的解析(HandlerMethodArgumentResolver), HandleMethod返回值的处理(HandlerMethodReturnValueHandler)，ModelAndView的构造
                 *          {@link AbstractHandlerMethodAdapter#handle(HttpServletRequest, HttpServletResponse, Object)}
                 * */
                // Actually invoke the handler.
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                // 是异步的，就直接结束
                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }

                /**
                 * 应用默认视图名称。如果没有指定viewName 默认就会使用范围路径作为视图名称
                 * {@link cn.haitaoss.controller.RequestMappingHandlerMapping.HelloController#test_default_view()}
                 * */
                applyDefaultViewName(processedRequest, mv);

                /**
                 * 回调 postHandle。
                 * 就是遍历 HandlerExecutionChain 中的 HandlerInterceptor 回调 {@link HandlerInterceptor#postHandle(HttpServletRequest, HttpServletResponse, Object, ModelAndView)}
                 * */
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            } catch (Exception ex) {
                // 记录异常，不需要抛出，下面会进行异常的处理
                dispatchException = ex;
            } catch (Throwable err) {
                // As of 4.3, we're processing Errors thrown from handler methods as well,
                // making them available for @ExceptionHandler methods and other scenarios.
                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            /**
             * 处理转发结果。
             *
             * 1. 存在异常，就遍历 HandlerExceptionResolver 处理异常。处理的结果会覆盖mv的值
             *      `mv = processHandlerException(request, response, handler, exception);`
             *      {@link DispatcherServlet#processHandlerException(HttpServletRequest, HttpServletResponse, Object, Exception)}
             *
             * 2. mv 不为null，就进行渲染
             *      `render(mv, request, response);`
             *      {@link DispatcherServlet#render(ModelAndView, HttpServletRequest, HttpServletResponse)}
             *      渲染视图，说白了拿到 View 示例，然后就将View实例对应的资源文件内容输出到response中，或者是重定向
             *
             * */
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        } catch (Exception ex) {
            /**
             * 回调 afterCompletion
             * 就是遍历 HandlerExecutionChain 中的 HandlerInterceptor 回调 {@link HandlerInterceptor#afterCompletion(HttpServletRequest, HttpServletResponse, Object, Exception)}
             * */
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        } catch (Throwable err) {
            // 回调 afterCompletion
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                    new NestedServletException("Handler processing failed", err)
            );
        } finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                if (mappedHandler != null) {
                    // 回调异步的拦截器。没用过，就不看了
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            } else {
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

    /**
     * Do we need view name translation?
     */
    private void applyDefaultViewName(HttpServletRequest request, @Nullable ModelAndView mv) throws Exception {
        if (mv != null && !mv.hasView()) {
            // 设置默认的视图名称。比如 /index 默认的名称就是 index
            String defaultViewName = getDefaultViewName(request);
            if (defaultViewName != null) {
                mv.setViewName(defaultViewName);
            }
        }
    }

    /**
     * Handle the result of handler selection and handler invocation, which is
     * either a ModelAndView or an Exception to be resolved to a ModelAndView.
     */
    private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
                                       @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
                                       @Nullable Exception exception) throws Exception {

        boolean errorView = false;

        // 处理异常情况
        if (exception != null) {
            if (exception instanceof ModelAndViewDefiningException) {
                logger.debug("ModelAndViewDefiningException encountered", exception);
                mv = ((ModelAndViewDefiningException) exception).getModelAndView();
            } else {
                Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
                /**
                 * 遍历 HandlerExceptionResolver 处理异常
                 * {@link DispatcherServlet#processHandlerException(HttpServletRequest, HttpServletResponse, Object, Exception)}
                 * */
                mv = processHandlerException(request, response, handler, exception);
                // 存在错误要展示的页面
                errorView = (mv != null);
            }
        }

        // 存在视图就进行渲染
        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) {
            /**
             * 渲染视图，说白了拿到 View 示例，然后就将View实例对应的资源文件内容输出到response中 或者是转发 或者是重定向
             * {@link DispatcherServlet#render(ModelAndView, HttpServletRequest, HttpServletResponse)}
             * */
            render(mv, request, response);
            if (errorView) {
                // 清除一些属性
                WebUtils.clearErrorRequestAttributes(request);
            }
        } else {
            if (logger.isTraceEnabled()) {
                logger.trace("No view rendering, null ModelAndView returned.");
            }
        }

        if (WebAsyncUtils.getAsyncManager(request)
                .isConcurrentHandlingStarted()) {
            // Concurrent handling started during a forward
            return;
        }

        if (mappedHandler != null) {
            // 回调 afterCompletion
            // Exception (if any) is already handled..
            mappedHandler.triggerAfterCompletion(request, response, null);
        }
    }

    /**
     * Build a LocaleContext for the given request, exposing the request's primary locale as current locale.
     * <p>The default implementation uses the dispatcher's LocaleResolver to obtain the current locale,
     * which might change during a request.
     *
     * @param request current HTTP request
     * @return the corresponding LocaleContext
     */
    @Override
    protected LocaleContext buildLocaleContext(final HttpServletRequest request) {
        /**
         * {@link AcceptHeaderLocaleResolver#resolveLocale(HttpServletRequest)}
         * */
        LocaleResolver lr = this.localeResolver;
        if (lr instanceof LocaleContextResolver) {
            return ((LocaleContextResolver) lr).resolveLocaleContext(request);
        } else {
            return () -> (lr != null ? lr.resolveLocale(request) : request.getLocale());
        }
    }

    /**
     * Convert the request into a multipart request, and make multipart resolver available.
     * <p>If no multipart resolver is set, simply use the existing request.
     *
     * @param request current HTTP request
     * @return the processed request (multipart wrapper if necessary)
     * @see MultipartResolver#resolveMultipart
     */
    protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
        if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
            if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
                if (DispatcherType.REQUEST.equals(request.getDispatcherType())) {
                    logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
                }
            } else if (hasMultipartException(request)) {
                logger.debug(
                        "Multipart resolution previously failed for current request - " + "skipping re-resolution for undisturbed error rendering");
            } else {
                try {
                    return this.multipartResolver.resolveMultipart(request);
                } catch (MultipartException ex) {
                    if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
                        logger.debug("Multipart resolution failed for error dispatch", ex);
                        // Keep processing error dispatch with regular request handle below
                    } else {
                        throw ex;
                    }
                }
            }
        }
        // If not returned before: return original request.
        return request;
    }

    /**
     * Check "javax.servlet.error.exception" attribute for a multipart exception.
     */
    private boolean hasMultipartException(HttpServletRequest request) {
        Throwable error = (Throwable) request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE);
        while (error != null) {
            if (error instanceof MultipartException) {
                return true;
            }
            error = error.getCause();
        }
        return false;
    }

    /**
     * Clean up any resources used by the given multipart request (if any).
     *
     * @param request current HTTP request
     * @see MultipartResolver#cleanupMultipart
     */
    protected void cleanupMultipart(HttpServletRequest request) {
        if (this.multipartResolver != null) {
            MultipartHttpServletRequest multipartRequest = WebUtils.getNativeRequest(
                    request, MultipartHttpServletRequest.class);
            if (multipartRequest != null) {
                this.multipartResolver.cleanupMultipart(multipartRequest);
            }
        }
    }

    /**
     * Return the HandlerExecutionChain for this request.
     * <p>Tries all handler mappings in order.
     *
     * @param request current HTTP request
     * @return the HandlerExecutionChain, or {@code null} if no handler could be found
     */
    @Nullable
    protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        /**
         * 是在web容器初始化Servlet时回调 {@link DispatcherServlet#initHandlerMappings(ApplicationContext)}
         * 完成属性的初始化。
         *
         * 默认是有这个三个：
         *      - BeanNameUrlHandlerMapping
         *      - RequestMappingHandlerMapping
         *      - RouterFunctionMapping
         * */
        if (this.handlerMappings != null) {
            for (HandlerMapping mapping : this.handlerMappings) {
                HandlerExecutionChain handler = mapping.getHandler(request);
                if (handler != null) {
                    return handler;
                }
            }
        }
        return null;
    }

    /**
     * No handler found -> set appropriate HTTP response status.
     *
     * @param request  current HTTP request
     * @param response current HTTP response
     * @throws Exception if preparing the response failed
     */
    protected void noHandlerFound(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (pageNotFoundLogger.isWarnEnabled()) {
            pageNotFoundLogger.warn("No mapping for " + request.getMethod() + " " + getRequestUri(request));
        }
        if (this.throwExceptionIfNoHandlerFound) {
            throw new NoHandlerFoundException(request.getMethod(), getRequestUri(request),
                    new ServletServerHttpRequest(request).getHeaders()
            );
        } else {
            response.sendError(HttpServletResponse.SC_NOT_FOUND);
        }
    }

    /**
     * Return the HandlerAdapter for this handler object.
     *
     * @param handler the handler object to find an adapter for
     * @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
     */
    protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        /**
         * 是在web容器初始化Servlet时回调 {@link DispatcherServlet#initHandlerAdapters(ApplicationContext)}
         * 完成属性的初始化。
         *
         * 默认是有这四个：
         *      - HttpRequestHandlerAdapter {@link HttpRequestHandlerAdapter#supports(Object)}
         *          handler instanceof HttpRequestHandler 才是true
         *
         *      - SimpleControllerHandlerAdapter {@link SimpleControllerHandlerAdapter#supports(Object)}
         *          handler instanceof Controller 才是true
         *
         *      - RequestMappingHandlerAdapter {@link AbstractHandlerMethodAdapter#supports(Object)}
         *          handler instanceof HandlerMethod 才是true
         *
         *      - HandlerFunctionAdapter {@link HandlerFunctionAdapter#supports(Object)}
         *          handler instanceof HandlerFunction
         * */
        if (this.handlerAdapters != null) {
            for (HandlerAdapter adapter : this.handlerAdapters) {
                if (adapter.supports(handler)) {
                    return adapter;
                }
            }
        }
        throw new ServletException(
                "No adapter for handler [" + handler + "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
    }

    /**
     * Determine an error ModelAndView via the registered HandlerExceptionResolvers.
     *
     * @param request  current HTTP request
     * @param response current HTTP response
     * @param handler  the executed handler, or {@code null} if none chosen at the time of the exception
     *                 (for example, if multipart resolution failed)
     * @param ex       the exception that got thrown during handler execution
     * @return a corresponding ModelAndView to forward to
     * @throws Exception if no error ModelAndView found
     */
    @Nullable
    protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
                                                   @Nullable Object handler, Exception ex) throws Exception {

        // Success and error responses may use different content types
        request.removeAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);

        // Check registered HandlerExceptionResolvers...
        ModelAndView exMv = null;
        /**
         * 是在web容器初始化Servlet时回调 {@link DispatcherServlet#initHandlerExceptionResolvers(ApplicationContext)}
         * 完成属性的初始化。
         *
         * 默认是有这三个：
         *  - ExceptionHandlerExceptionResolver
         *  - ResponseStatusExceptionResolver
         *  - DefaultHandlerExceptionResolver
         * */

        if (this.handlerExceptionResolvers != null) {
            for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
                // 使用异常处理器解析异常                
                exMv = resolver.resolveException(request, response, handler, ex);
                // 不会null 说明异常已经被处理，不需要下一个解析器解析了
                if (exMv != null) {
                    break;
                }
            }
        }
        if (exMv != null) {
            // 没有指定view 且 没有往Model属性
            if (exMv.isEmpty()) {
                //将异常对象存到request域中
                request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
                // 返回这个说明，不需要视图的渲染，我们应当自己往Response中写东西，否则客户端会发现这次请求的response是空的
                return null;
            }
            // 没有指定 view 设置默认名称
            // We might still need view name translation for a plain error model...
            if (!exMv.hasView()) {
                String defaultViewName = getDefaultViewName(request);
                if (defaultViewName != null) {
                    exMv.setViewName(defaultViewName);
                }
            }
            if (logger.isTraceEnabled()) {
                logger.trace("Using resolved error view: " + exMv, ex);
            } else if (logger.isDebugEnabled()) {
                logger.debug("Using resolved error view: " + exMv);
            }
            // 将异常信息设置到 request 域中
            WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
            return exMv;
        }

        throw ex;
    }

    /**
     * Render the given ModelAndView.
     * <p>This is the last stage in handling a request. It may involve resolving the view by name.
     *
     * @param mv       the ModelAndView to render
     * @param request  current HTTP servlet request
     * @param response current HTTP servlet response
     * @throws ServletException if view is missing or cannot be resolved
     * @throws Exception        if there's a problem rendering the view
     */
    protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
        /**
         * 拿到 Locale
         *
         * 默认是 {@link AcceptHeaderLocaleResolver#resolveLocale(HttpServletRequest)}
         * */
        // Determine locale for request and apply it to the response.
        Locale locale = (this.localeResolver != null ? this.localeResolver.resolveLocale(
                request) : request.getLocale());
        response.setLocale(locale);

        View view;
        String viewName = mv.getViewName();
        if (viewName != null) {
            /**
             * 遍历视图解析器，先解析到view就返回
             * View最关键的就是有ViewName其实就是资源文件的路径，相当于将资源文件(html) 映射成 View 对象
             * */
            // We need to resolve the view name.
            view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
            if (view == null) {
                throw new ServletException(
                        "Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + getServletName() + "'");
            }
        } else {
            // No need to lookup: the ModelAndView object contains the actual View object.
            view = mv.getView();
            if (view == null) {
                throw new ServletException(
                        "ModelAndView [" + mv + "] neither contains a view name nor a " + "View object in servlet with name '" + getServletName() + "'");
            }
        }

        // Delegate to the View object for rendering.
        if (logger.isTraceEnabled()) {
            logger.trace("Rendering view [" + view + "] ");
        }
        try {
            // 设置状态码
            if (mv.getStatus() != null) {
                response.setStatus(mv.getStatus()
                        .value());
            }
            // 渲染视图。就是将View对应的资源文件输出的response中
            view.render(mv.getModelInternal(), request, response);
        } catch (Exception ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Error rendering view [" + view + "]", ex);
            }
            throw ex;
        }
    }

    /**
     * Translate the supplied request into a default view name.
     *
     * @param request current HTTP servlet request
     * @return the view name (or {@code null} if no default found)
     * @throws Exception if view name translation failed
     */
    @Nullable
    protected String getDefaultViewName(HttpServletRequest request) throws Exception {
        return (this.viewNameTranslator != null ? this.viewNameTranslator.getViewName(request) : null);
    }

    /**
     * Resolve the given view name into a View object (to be rendered).
     * <p>The default implementations asks all ViewResolvers of this dispatcher.
     * Can be overridden for custom resolution strategies, potentially based on
     * specific model attributes or request parameters.
     *
     * @param viewName the name of the view to resolve
     * @param model    the model to be passed to the view
     * @param locale   the current locale
     * @param request  current HTTP servlet request
     * @return the View object, or {@code null} if none found
     * @throws Exception if the view cannot be resolved
     *                   (typically in case of problems creating an actual View object)
     * @see ViewResolver#resolveViewName
     */
    @Nullable
    protected View resolveViewName(String viewName, @Nullable Map<String, Object> model, Locale locale,
                                   HttpServletRequest request) throws Exception {

        /**
         * 是在web容器初始化Servlet时回调 {@link DispatcherServlet#initViewResolvers(ApplicationContext)}
         * 完成属性的初始化。
         *
         * 默认是有这一个：
         *    {@link InternalResourceViewResolver}
         * */
        if (this.viewResolvers != null) {
            for (ViewResolver viewResolver : this.viewResolvers) {
                /**
                 * 解析值 不是null 就说明解析到了。
                 *
                 * 解析，其实就是 new View() 实例
                 * view最主要的属性是 url。url就可以根据视图解析器的前缀后缀拼接出来，其实就是资源文件的路径
                 * */
                View view = viewResolver.resolveViewName(viewName, locale);
                if (view != null) {
                    return view;
                }
            }
        }
        return null;
    }

    private void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response,
                                        @Nullable HandlerExecutionChain mappedHandler, Exception ex) throws Exception {

        if (mappedHandler != null) {
            mappedHandler.triggerAfterCompletion(request, response, ex);
        }
        throw ex;
    }

    /**
     * Restore the request attributes after an include.
     *
     * @param request            current HTTP request
     * @param attributesSnapshot the snapshot of the request attributes before the include
     */
    @SuppressWarnings("unchecked")
    private void restoreAttributesAfterInclude(HttpServletRequest request, Map<?, ?> attributesSnapshot) {
        // Need to copy into separate Collection here, to avoid side effects
        // on the Enumeration when removing attributes.
        Set<String> attrsToCheck = new HashSet<>();
        Enumeration<?> attrNames = request.getAttributeNames();
        while (attrNames.hasMoreElements()) {
            String attrName = (String) attrNames.nextElement();
            if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                attrsToCheck.add(attrName);
            }
        }

        // Add attributes that may have been removed
        attrsToCheck.addAll((Set<String>) attributesSnapshot.keySet());

        // Iterate over the attributes to check, restoring the original value
        // or removing the attribute, respectively, if appropriate.
        for (String attrName : attrsToCheck) {
            Object attrValue = attributesSnapshot.get(attrName);
            if (attrValue == null) {
                request.removeAttribute(attrName);
            } else if (attrValue != request.getAttribute(attrName)) {
                request.setAttribute(attrName, attrValue);
            }
        }
    }

    private static String getRequestUri(HttpServletRequest request) {
        String uri = (String) request.getAttribute(WebUtils.INCLUDE_REQUEST_URI_ATTRIBUTE);
        if (uri == null) {
            uri = request.getRequestURI();
        }
        return uri;
    }

}
